Systems and method for boosting output of an alternator

ABSTRACT

Disclosed herein are two techniques, neutral point switching and field voltage boost, that will increase the output of today&#39;s 12 volt automotive electrical systems in vehicle idle conditions solely by the addition of circuitry. Neutral point switching enables the flow of a third harmonic current, which does not normally flow at low speeds, but only at high speed. Boosting the field voltages can be obtained by integrating a field voltage boost circuit and voltage regulator to increase the field voltage, and consequently the field current, above the level obtained from the battery. Furthermore, the transient response of the alternator to a change in load is improved by temporarily increasing the field voltage above the level needed to sustain the load. These two techniques are compatible, and thus may be implemented together, or may be implemented independently. No changes to a standard alternator are required to accommodate the proposed additional circuitry.

FIELD OF THE INVENTION

This invention relates to automotive electrical systems. Moreparticularly, the invention relates to configurations of automotiveelectrical power systems adapted for use with high power loads.

BACKGROUND

The 12 volt systems used in today's automobiles are required to supplyever increasing currents as the load on the system continues toincrease. This increase is due to a combination of increasing numbers ofelectronic devices, such as communication, entertainment, and telematicssystems, as well as the proliferation of electric powered auxiliarysystems to replace traditional hydraulic or mechanical powered systems.To reduce the amount of current required to supply these higher loads,it has been proposed that automobiles should adopt 42 volt electricalsystems. However, the automotive industry has been reluctant totransition to 42 volt electrical systems because of increased costs.Consequently, there is a strong demand to improve the performance of 12volt systems, thereby allowing higher electrical loads to operateeffectively with conventional vehicle electrical systems.

One principal limitation in the performance of automotive electricalsystems is the alternator, and particularly the amount of current thatcan be drawn from the alternator and the response time required to drawthis current. The output current capability of today's automotivealternator is influenced by the speed at which the alternator isoperating, which is determined by the engine speed of the vehicle. Atypical alternator might produces a rated current of 135 amperes at anengine speed of 3000 rpm, might typically produces only 60 amperes at anengine speed of 600 rpm (corresponding to engine idle). Most automotiveelectrical loads are insensitive to vehicle speed, such as rear windowdefoggers, heated seats, lights, HVAC blowers, entertainment devices,etc. The loads that are sensitive to engine speed (e.g., ignition) donot consume significant current. Consequently, the electrical system isin significant current deficit at idle, which can produce voltagefluctuations if additional current is required. Furthermore, becausesome loads are very sensitive to voltage fluctuations (e.g., lights), analternator that responds quickly to large load application will reducethe undesirable effects of voltage fluctuations, such as lightflickering, seen by the driver.

It bears mentioning that the problem addressed herein is not one ofpower limitation where the alternator cannot supply sufficient power,but rather is a problem of voltage limitation. Fundamentally, the backemf (electromotive force) produced by the alternator is not large enoughto supply the required current at engine idle, even with the fieldcurrent at its max value.

Solutions to address certain aspects of the performance deficiencies inautomotive electrical systems are addressed in co-pending U.S. patentapplication, having the Ser. No. 60/599,328, filed Aug. 6, 2004, andentitled “Automotive Electrical System Configuration.” This patentapplication is hereby incorporated by reference in its entirety. Thepresent invention attempts to further minimize the above-mentioneddrawbacks and proposes a system that solves or at least minimizes theproblems of the prior art.

SUMMARY OF THE INVENTION

Disclosed herein are two techniques, neutral point switching and fieldvoltage boost, that will increase the output of today's 12 voltautomotive electrical systems in vehicle idle conditions solely by theaddition of circuitry. Neutral point switching enables the flow of athird harmonic current, which does not normally flow at low speeds, butonly at high speed. Boosting the field voltages can be obtained byintegrating a field voltage boost circuit and voltage regulator toincrease the field voltage, and consequently the field current, abovethe level obtained from the battery. Furthermore, the transient responseof the alternator to a change in load is improved by temporarilyincreasing the field voltage above the level needed to sustain the load.These two techniques are compatible, and thus may be implementedtogether, or may be implemented independently. No changes to a standardalternator are required to accommodate the proposed additionalcircuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the inventive aspects of this disclosure will be bestunderstood with reference to the following detailed description, whenread in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a prior art circuit used in some automobile chargingsystems having a pair of diodes connected across the alternator'sneutral terminal.

FIG. 2 illustrates a variation of the circuit of FIG. 1 in which thediodes are replaced with field effect transistors (FETs).

FIG. 3A illustrates yet another variation of the circuit of FIGS. 1 and2 further comprising a buck converter connected between thebattery/alternator side of the bus, i.e., the supply side of the bus,and the load side of the bus.

FIG. 3B illustrates a further modification of the circuit of FIG. 3A inwhich an additional buck converter is provided for the negative DC bus.

FIGS. 3C–3E illustrate typical waveforms of the circuit illustrated inFIG. 3B.

FIG. 4 illustrates a field voltage boost circuit in accordance withcertain teachings of the present disclosure.

FIG. 5 is a plot of field current and voltage versus time for a loadcurrent increase.

FIG. 6 illustrates a circuit in which neutral point switching iscombined with field voltage boost.

DETAILED DESCRIPTION

As noted above, the present disclosure is directed to techniques forimproving the response of an automotive-type electrical system, and moreparticularly for improving alternator performance. A typical prior artautomotive electrical system is illustrated in FIG. 1. The electricalsystem comprises a battery 101, which stores electrical energy for usewhen the vehicle is not running and when alternator 102 is unable tosupply the full amount of electrical energy required by loads 106.

Under normal, steady state, operating conditions alternator 102, drivenvia a belt by the vehicle engine (not shown) generates the electricalenergy required by loads 106. However, because the rest of theelectrical system is DC, the AC voltage produced by alternator 102 mustbe rectified using rectifier bank 104. As illustrated rectifier bank 104is a three phase full wave diode bridge, but other forms of rectifiers,including those using switched semiconductors such as silicon controlledrectifiers or transistors (e.g., FETs) could be used. Regulator 103 isused to control the field voltage of alternator 102, and consequentlythe field current, to regulate the voltage ultimately produced on thepositive DC bus 109 of the electrical system. Design and operation ofsuch regulators is well known in the art, and details may be found in“Automotive Electrics and Electronics”, Bosch Automotive Handbook,3^(rd) Edition, pp. 138–142, which is incorporated by reference in itsentirety.

In some automotive electrical systems, a pair of diodes 107 is providedconnecting at their junction to the neutral lead of alternator 102 toprovide additional charging current at high engine speeds. The extracurrent results from third harmonic current generated because the outputvoltage of most alternators is approximately a square wave. Square wavealternators are used because it is cheaper and more efficient tomanufacture a square-wave alternator as opposed to a pure sine wavealternator. The square wave waveform contains measurable third harmonicvoltage, which does not contribute to the alternator output if thetypical six diode bridge is used. (The third harmonic voltages for eachalternator leg are in phase with each other and thus there is nodifferential third harmonic voltage difference between any two of threephases. However, these voltages are measurable between phase and neutralpoints.) Thus an additional pair of diodes 107 may be added, with thediode pair center point connected to the alternator neutral 108, asshown in FIG. 1. This diode pair does not conduct until the amplitude ofthe third harmonic is large enough to forward bias the diodes. Thisoccurs at relatively high engine speeds and does not contributesignificantly to the charging current at idle. When the amplitude of thethird harmonic is large enough to forward bias a diode pair, one of thediodes in the neutral leg conducts, say the upper diode, while thereturn path is one of the lower three diodes of the conventional sixdiode rectifier bridge. When the third harmonic voltage switchespolarity, the lower neutral diode conducts and one of the three upperconventional diodes provides the return path.

One technique for increasing the current drawn from today's 12 voltalternator at vehicle idle is to add an additional circuit to thealternator neutral point. This circuit should draw current from theneutral point at idle that would otherwise not be generated. Thiscircuit is not to be confused with the additional diode pair connectedto the neutral which only adds to the charging current at highalternator speeds. The circuit needed, illustrated in FIG. 2, replacesthis auxiliary diode pair 107 with a pair of FETs 207 that can beactively switched to force a third harmonic current to flow at idle. Thecontrol scheme uses a square wave to control the switching of the FETpair. Maximum third harmonic current will be drawn from the alternatorif this square wave is 90 degrees out of phase with the neutral voltage,assuming zero alternator winding resistance. In reality, this resistancemust be included and the actual angle depends on the value of alternatorresistance, alternator inductance, alternator pole count and alternatorspeed. Alternatively, the current drawn may be modulated by replacingthe square wave with a PWM (pulse width modulated) waveform.

For the remainder of the discussion, it will be assumed that the maximumcurrent is required, and the phase of the square wave used to drive theauxiliary FETs 207 is controlled to produce maximum current. (Techniquesfor determining the proper square wave phase, such as lookup tables orsearcher algorithms are described below.) The current drawn from thebattery as a function of battery voltage is shown in Table 1. The firstcolumn indicates the battery voltage. The second column gives theneutral current using only diodes 107 as illustrated in FIG. 1. Thethird column gives neutral current using FETs 207 as illustrated in FIG.2.

TABLE 1 neutral switches Battery no neutral with neutral and buckvoltage switches switches (Vbuck = 15) 13 V 48.1 A 48.8 A 58.5 A 14 V44.3 A   47 A 51.3 A

The neutral current drawn from the alternator may be further enhanced bydisconnecting the upper FET from the battery voltage (i.e., positive DCbus 109) and connecting it to an intermediate voltage that is higherthan battery voltage. This configuration is shown in FIG. 3A. Theintermediate voltage is regulated by buck converter 301, which bucks theintermediate voltage of bus 109 b down to the typical bus voltage ofpositive DC bus 109 a. The voltage of DC bus 109 a is regulated to anappropriate voltage for battery charging.

The voltage-time product for auxiliary switches 207 must be balanced sothe lower switch of the pair is conducting for more than 50% of thevoltage-time product, while the upper switch conducts less than 50%.Otherwise, the third harmonic current drawn from the alternator willhave a DC component, leading to reduced overall current output as thealternator saturates. The current drawn for this enhanced circuit isshown in the fourth column of Table 1.

Another variation of the neutral current switching circuit isillustrated in FIG. 3B. This circuit is similar to that of FIG. 3A, buta second buck converter has been added in the negative voltage bus 303 ato lower this voltage of bus 303 b below the voltage of negative voltagebus 303 a. Additionally, the loads are now supplied from the normal bus,and the 109 b/303 b bus is essentially a short term storage bus becauseof capacitors 302. In this case, the voltage seen by the alternator isbalanced and a 50% square wave is used to control the operation of theneutral switch pair.

Typical waveforms of the FIG. 3B circuit are shown in FIGS. 3C, 3D, and3E. FIG. 3C shows the back emf waveforms for each phase 304, 305, and306. FIG. 3C also shows the 20% third harmonic component 307. FIG. 3Dshows the current 308 into the battery, which is a sum of the currentfrom the diode bridge and the double buck converters. FIG. 3Eillustrates the relationship between the third harmonic phase 307 andthe phase of the switching square wave 309. In this case, the phasedifference is almost 180 degrees. This relatively large phase differenceis because the winding resistance of the alternator is included in themodel. The alternator parameters used in the circuit model are:inductance of 105 μH, resistance of 33 mΩ, speed 1800 rpm, 6 pole pairstator alternator winding.

The neutral point FETs are switched with 50% duty cycle at the frequencyof the third harmonic. A shaft position sensor, such as a resolver, maybe used to determine shaft position so that the frequency and phase ofthe 50% square wave is synchronized with the frequency and phase of thethird harmonic voltage. The phase angle of the switching square wavemust lag the phase angle of the third harmonic component to compensatefor the phase shift that occurs across the alternator impedance(inductance and resistance). The required phase difference will, ofcourse, vary as a function of alternator current. One way to provide theswitching circuit with the appropriate phase shift values is tocalculate it for certain current values (based on alternator resistanceand inductance) and provide a lookup table in the neutral switchcontroller. Alternatively, the phase shift may be determined on-line bya searcher algorithm that continuously varies the phase until maximumpower is obtained.

An alternative technique to neutral point switching that will alsoincrease the alternator output is to increase the field current into thealternator. The regulators used in conventional automotive electricalsystems are not capable of increasing the field current above the limitestablished by the battery voltage and field resistance. This limitationmay be overcome by adding a boost converter circuit to increase thevoltage into the regulator above the battery voltage. This consequentlyincreases the maximum possible field current.

Such a circuit is illustrated in FIG. 4. A field voltage boost circuit402 is placed between the positive DC bus 109 and the “top” of thealternator field winding, as illustrated. (Capacitor 403 smoothes thevoltage out of boost circuit 402.) This allows boost circuit 402 toincrease the maximum voltage applied to the alternator's field winding,which allows greater field current than can be achieved with batteryvoltage alone. The voltage profile output by boost circuit 402 isintended to overcome the relatively large time constant associated withthe field winding (300 ms is a typical number). It is anticipated that aboost to the field winding voltage will be required by the activation ofa large load (e.g., electric power steering or “EPS”), and thus thecurrent drawn from the battery increases proportionally. Thus thebattery current is used as an input to boost control circuit 404 as afeedforward signal. It may be more beneficial to use the current drawnfrom the large loads as a feedforward signal, depending on the systemconfiguration and parameters.

For relatively small load changes, the boost is activated when theregulator duty cycle is greater than a predetermined threshold (95%, forexample). The boost is turned off when the regulator duty cycle fallsbelow a second predetermined threshold less than the first predeterminedthreshold (90%, for example). This hysteresis is desirable to preventthe boost converter from interacting with the dynamics of the voltageregulator. Once the boost circuit is activated, the regulator duty cycleis maintained within the upper and lower bounds by a control loop whichdetermines how the boost voltage is modified. For example, at a periodicinterval, e.g., 1 ms, the regulator duty cycle is determined. If theduty cycle exceeds the upper bound the boost voltage is increased by0.1V. However, if the duty cycle is less than the lower bound, the boostvoltage is decreased by 0.1V. In this manner, the regulator duty ismaintained with the desired bounds. Other techniques may also be used toregulate the boost output voltage. For example, a PI(proportional-integral) loop may be used to regulate the boost outputvoltage to the average of the upper and lower bounds.

As noted above, the battery current is monitored to determine when alarge change in load occurs. The regulator PWM parameters are tuned sothat small load changes will result in suitable field voltage changes.However, in the case of a sufficiently large load transient, theregulator will attempt to establish a PWM duty cycle exceeding 100%, andthus the regulator alone cannot be used to determine field voltage. Inthis case, the battery current is used to generate an estimate of thenew field current required and a final boost output voltage (V_(final))is determined to re-establish operation of the regulator within the dutycycle range determined by the predetermined thresholds, (e.g., the90%–95% range).

The field voltage is regulated according to:V _(f) =K _(p)·(13.5−V _(alt))+K ₁·∫(135−V _(alt))+K _(FF) ·I _(bat)where V_(f) is the field voltage, K_(P) and K_(I) are the proportionaland integral gains of a well-known PI (proportional-integral)controller, V_(alt) is the alternator voltage, K_(FF) is the feedforward gain and I_(bat) is the battery current. Selection of anappropriate feed forward gain for the battery current will lead to anincrease or decrease in field voltage before the output voltage changes.Design techniques for these controllers are generally known to thoseskilled in the art, and may also be found in “Computer ControlledSystems: Theory and Design”, by Astrom/Wittenmark, 1990, pp. 150–151(which is incorporated by reference). The feedforward gain, K_(ff), maybe varied as a function of field current if the alternator rotor is insaturation. When the rotor is in saturation, an increase in fieldcurrent results in a smaller increase in back emf and a correspondinglysmaller increase in battery charging current, i.e., diminishing returns.The saturation phenomenon is explained in “Electric Machinery”, byFitzgerald et.al., 1983, p. 176–178, which is incorporated by reference.When the machine is saturated, a plot of field current versus opencircuit voltage shows a deviation from a straight line. As the fieldcurrent increases and saturation begins, the constant slope reduces asthe output voltage increase in less than the field current increase.Ideally, K_(ff) is modified so that the product of K_(ff) and theinverse of the open circuit curve slope is a constant. In effect, K_(ff)increases at the onset of saturation and continues to increase as theamount of saturation increases.

Transient performance of the circuit of FIG. 4 may be further enhancedby increasing the boost voltage above V_(final), as shown in FIG. 5.This decreases the time required for the field current to reach itsfinal value. In this case, the time for which this additional boostvoltage is applied can be calculated as illustrated in the equationsbelow. The final value of field current is given by:

$I_{f\_ final} = {I_{f\_ init} + {\frac{V_{extra}}{R_{f}}\left( {1 - {\mathbb{e}}^{\frac{- t}{\tau}}} \right)}}$where I_(f) _(—) _(final) is the final field current, I_(f) _(—) _(init)is the initial field current, V_(extra) is the temporary boost voltage,R_(f) is the field winding resistance, and τ is the alternator timeconstant. Rearranging this equation, the time t becomes:

$t = {\ln\left( \frac{V_{extra}}{V_{extra} - {\left( {I_{f\_ final} - I_{f\_ init}} \right) \cdot R_{f}}} \right)}$For example, if V_(extra)=30V, R_(f)=3Ω, I_(f) _(—) _(delta)=1A, τ=300ms, then t=32 ms. The above calculation illustrates how the responsetime of the alternator is increased by temporarily increasing the fieldvoltage above its final value. In practice, it may be necessary toinclude the effects of alternator field saturation, which will slow thealternator response somewhat. This calculation must be performed on acase by case basis depending on the parameters of the particularalternator chosen.

When load is removed, normal regulator operation is used to reduce theregulator PWM while the boost voltage is gradually reduced to bring thePWM back to the 90% to 95% range. The dynamics in this case are the sameas for a small increase in load current. Of course, the load may havedropped low enough that when the boost is shut off the regulator PWMdoes not exceed 90%.

FIG. 6 illustrates a circuit in which neutral point switching iscombined with field voltage boost. Operation of this circuitincorporates both concepts and further improves alternator transientresponse.

It should be understood that the inventive concepts disclosed herein arecapable of many modifications, combinations and subcombinations.Furthermore, the block diagram elements shown in the figures are meantto illustrate the inventive concepts described herein and are notintended to be complete circuit diagrams. It is intended that thispatent be afforded the full scope of the appended claims and theirequivalents.

1. An automotive electrical system comprising: a storage battery havinga positive terminal connected to a positive bus and a negative terminalconnected to a negative bus; an alternator having its output connectedto a rectifier bank, wherein the output terminals of the rectifier bankare connected to the positive bus and the negative bus; a voltageregulator configured to control a field current of the alternator inresponse to the voltage between the positive bus and the negative bus;and a field voltage boost circuit connected to an input of the voltageregulator and configured to increase the maximum voltage available to afield winding so as to increase the field current of the alternator. 2.The automotive electrical system of claim 1 wherein the field voltageboost circuit is controlled in response to a current through thebattery.
 3. The automotive electrical system of claim 2 wherein thefield voltage boost circuit is further controlled in response to a dutycycle of the voltage regulator.
 4. The automotive electrical system ofclaim 3 wherein the field voltage boost circuit is engaged when the dutycycle of the voltage regulator exceeds a predetermined threshold.
 5. Theautomotive electrical system of claim 4 wherein the predeterminedthreshold corresponds to a duty cycle of 95%.
 6. The automotiveelectrical system of claim 4 wherein the voltage boost circuit isdisengaged when the duty cycle of the voltage regulator drops below asecond predetermined threshold.
 7. The automotive electrical system ofclaim 6 wherein the second predetermined threshold corresponds to a dutycycle of 90%.
 8. The automotive electrical system of claim 1 furthercomprising a neutral switch comprising a first switching device and asecond switching device, wherein the first switching device is connectedbetween a neutral terminal of the alternator and the positive bus andwherein the second switching device is connected between the neutralterminal of the alternator and the negative bus, the neutral switchbeing configured to allow third harmonic current generated by thealternator to flow to one or more loads connected between the positivebus and the negative bus.
 9. The automotive electrical system of claim 8further comprising a buck converter connected between the positive busand a second positive bus such that the buck converter controls thevoltage of the first positive bus to effect charging of the battery, andwherein the first switching device is connected between the alternatorneutral terminal and the second positive bus.
 10. The automotiveelectrical system of claim 8 further comprising a second buck converterconnected between the negative bus and a second negative bus such thatthe buck converter reduces the voltage of the second negative busrelative to the negative bus and wherein the second switching device isconnected between the alternator neutral terminal and the secondnegative bus.